_T. rex_ is "brainier" than crocs (and most herbivorous dinosaurs), but
that isn't saying much. Most of _T. rex_'s brain volume was taken up by
the animal's sensory subsystems (olfactory lobes, and to a lesser extent,
its optic lobes). But for complex social interactions, a larger cortex
is needed and _T. rex_ just doesn't measure up in that Department.

The sheer complexity of intra-species interactions such as that
exemplified by the ecological niche of the cleaner wrass suggests that
brain size is not a good indication of the presence or absence of
complex social interactions. In this example, several species of fish
have to temporarily modify their natural instincts in order to take
advantage of the services provided by cleaner wrass. The wrass
themselves also have to be able to interpret the signals given by
other species in order to approach without fear.

Great White sharks engage in complex social interactions with one
another, communicating their feelings via body posture. They've been
shown to recognise familiar animals by sight, and exhibit signs of
stress when a strange shark gets too close. They communicate their
nervousness by slightly arching their backs and dropping their
pectoral fins down, which warns a stranger not to get too close.

Complex social behaviour needn't necessarily be 'conscious' social
behaviour, hence it doesn't always require a large cortex.